The use of phase modulated optical signals in optical communication systems offers considerable advantages over on-off keying of optical signals intensity in many cases. Examples of phase modulation formats that have attracted particular attention in optical applications include various phase-shift keying (PSK) modulation formats, such as binary PSK (BPSK) and quadrature PSK (QPSK). However, demodulating such signals is more complicated than detecting and demodulating intensity modulated signals. One known phase demodulation scheme includes the use of delay line interferometers, in which an input PSK modulated optical signal is coherently combined with a copy thereof that is delayed by one PSK symbol. One disadvantage of such schemes is that they are not bit rate flexible, i.e. the fixed delay line only decodes signals modulated at a certain bit rate. Furthermore, the delay lines may have to be rather long; for example, a 1 Gbit/s signal requires a delay line that is about 20 cm long, which is not practical in small form factor components.
Another known PSK decoding schemes uses optical homodyne detection, in which light from a local oscillator (LO) laser is mixed with the PSK modulated light of a transmitter laser. Both the LO laser and the transmitter laser can be made stable, and the LO laser can be optically locked to the transmitter laser. This solution is however not very cost efficient.
In a conventional scheme of optical coherent homodyne reception the detected signals from one or more photodiodes are digitized, and the phase detection is done in a digital signal processor (DSP). However, sufficiently fast digitizers and DSPs may be expensive and power hungry. In some schemes with optical LO phase locking, the control signal of a phase tracking circuit itself can be used as a demodulated signal. This however works only for limited data rates.
In the case of coherent heterodyne reception, e.g., when wavelengths of the LO and transmitter lasers differ, the tracking of the phase can be done in the electric domain instead of the optical domain. Known techniques however typically require a fast tunable voltage controlled oscillator (VCO), which may be expensive; furthermore, designing a sufficiently robust control loop using a VCO is a non-trivial task.
Accordingly, it may be understood that there may be significant problems and shortcomings associated with current solutions and technologies for demodulating PSK modulated electrical and optical signals, including those used in high-speed optical communication systems.